Secondary solar concentrator

ABSTRACT

An external concentrator is provided for use with heat collection elements (HCE&#39;s of a solar parabolic trough power plant. In one arrangement, the concentrator includes a plurality of ribs that are adapted to extend radially outward from the outside surface of an HCE and along the linear length of the HCE to help redirect stray/spilled light into the absorber tube of the HCE. In another arrangement, the concentrator includes a shield placed on or near a surface of the HCE opposite of the parabolic reflective trough. The reflective shield includes ribs or brims that are disposed adjacent to one or both lateral edges of a reflective shield applied to the outside surface of a HCE tube to increase the collection of stray light reflected by the reflective trough.

CROSS REFERENCE

The present application claims the benefit of the filing date of U.S. Provisional Application No. 62/346,020 having a filing date of Jun. 6, 2016, the entire contents of which are incorporated herein by reference.

FIELD

The present disclosure is directed to improving the performance of parabolic trough solar collectors. More specifically, the present disclosure is directed to a secondary solar concentrator that improves concentration of beam radiation onto tubular receivers or heat collection elements (HCE's) of parabolic trough solar collectors.

BACKGROUND

A parabolic trough power plant generates electricity using concentrated sunlight as the heat source for its power cycle. Most commonly rows of single-axis-tracking, linear parabolic mirrors form a solar field that concentrate beam radiation onto tubular receivers which are also known as heat collection elements (HCE's). See, e.g., FIG. 1A. The HCE's are located along the focal line of each parabolic trough. Heat-transfer fluid pumped through the HCE's is heated by the sun heated receiver walls on which the parabolic mirrors focus solar radiation. See, e.g., FIG. 1B. After being heated by the solar field, the heat-transfer fluid is typically used generate high-pressure superheated steam in a series of heat exchangers. Most commonly, the energy in the steam is converted to electricity in a Rankine steam turbine power cycle. After passing through the heat exchangers, the cooled heat transfer fluid is recirculated through the solar HCE's.

SUMMARY

Aspects of the presented inventions are based on the recognition by the inventor that the focal point of linear parabolic reflectors/mirrors is often not exact. That is, the consistency of the actual foci of the parabolic mirrors as it focuses light onto the HCE's is somewhat loose in tolerance. Along these lines, a portion of the beam radiation reflected by the mirrors may not contact the heat collection elements mounted along the foci of the linear parabolic reflectors. Stated otherwise, some of the reflected beam radiation is lost via spillage. The reflected beam radiation which never contacts an HCE is lost energy, which could be utilized to further heat the heat-transfer fluid and further improve overall efficiency of the system. To reduce such spillage, the presented inventions are directed to a secondary solar concentrator that may be affixed about an external surface of an existing HCE to capture and redirect reflected beam radiation that would otherwise bypass the HCE.

In one aspect, an external concentrator includes a plurality of ribs that are adapted to extend radially outward from the outside surface of an HCE and along the linear length of the HCE to help redirect stray/spilled light into the absorber tube of the HCE. The number and spacing of the ribs may be varied. In any arrangement, the ribs form a reflective surface that allows for redirecting stray light into the HCE.

In a further arrangement, the external concentrator includes two sets of ribs that are disposed on different radial sections of the outside surface of the HCE. In such an arrangement, the different sets of ribs may be separated by a reflective shield that covers a portion of the HCE tube. Most commonly, when applied to an HCE tube, the reflective shield is disposed outside of the tube opposite of the vertex of a parabolic reflector that focuses light onto the tube.

In another arrangement, the external concentrator includes two sets of ribs that are disposed on different radial sections of the outside surface of the HCE. In such an arrangement, individual ribs may be disposed in non-radial orientations relative to the surface of the HCE, and different orientations relative to other ribs.

In yet another arrangement, the external concentrator may include ribs or brims that are disposed adjacent to one or both lateral edges of a reflective shield applied to the outside surface of a HCE tube. In such an arrangement the ribs/brims may connect to the reflective shield at a pivot point and be disposed in various angular orientations relative to the surface of the HCE. Such ribs/brims may extend above and outward from the surface of the HCE to collect additional stray light. Certain embodiments also contemplate spacing the reflective shield and/or the ribs at a distance from the surface of the HCE.

In another aspect, a method is provided for retrofitting existing parabolic trough power plants to increase efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a solar field formed of a plurality of parabolic reflectors.

FIG. 1B illustrates a cross-sectional view of parabolic reflector focusing solar energy on a focal point.

FIG. 2 illustrates one embodiment of a heat collecting element.

FIG. 3A illustrates spillage of reflected light at a heat collecting element.

FIG. 3B illustrates an end view of an external concentrator.

FIG. 3C illustrates an end view of an external concentrator as applied to the heat collecting element.

FIG. 3D illustrates the external concentrator of FIG. 3C redirecting spilled light the FIG. 3A onto the heat collecting element.

FIGS. 4A and 4B illustrate first and second perspective views of an external concentrator as applied to a heat collecting element.

FIGS. 5A and 5B illustrate first and second and views of an external concentrator.

FIG. 6 illustrates an external concentrator embodiment with ribs disposed in non-radial orientations.

FIG. 7 illustrates an external concentrator with ribs disposed at end portions of the reflective shield.

FIGS. 8A and 8B illustrate first and second embodiments of an external collector oriented in a spaced relationship relative to the heat collecting element.

DETAILED DESCRIPTION

Reference will now be made to the accompanying drawings, which assist in illustrating the various pertinent features of the presented inventions. The following description is presented for purposes of illustration and description and is not intended to limit the inventions to the forms disclosed herein. Consequently, variations and modifications commensurate with the following teachings, and skill and knowledge of the relevant art, are within the scope of the presented inventions. The embodiments described herein are further intended to explain the best modes known of practicing the inventions and to enable others skilled in the art to utilize the inventions in such, or other embodiments and with various modifications required by the particular application(s) or use(s) of the presented inventions.

FIG. 1A illustrates an exemplary parabolic trough solar assembly having a plurality of linear parabolic mirrors/reflectors 10. As shown, the linear parabolic reflectors 10 are disposed in rows and each row of reflectors is operative to concentrate or focus solar radiation onto a receiver tube(s) or heat collection element(s) 20 (HCE) linearly disposed along the linear focal lines/points of the reflectors 10. See FIG. 1B showing a cross-section of a parabolic trough 10 with an ideal focal point at the HCE 20. That is, the reflectors are oriented such that sunlight which they reflect concentrates on the HCE, which contains a circulating heat transfer fluid that is heated to a high temperature by the energy of the concentrated sunlight. The heat transfer fluid (often thermal oil) runs through the HCE to absorb the concentrated sunlight. This increases the temperature of the fluid to, in some cases, 400° C. The heat transfer fluid is then most commonly used to heat steam in a turbine generator. Other thermal uses are possible.

FIG. 2 illustrates one non-limiting embodiment of an HCE 20. The illustrated embodiment of the HCE 20 is shortened for purposes of illustration. However, it will be appreciated that such HCE's may be of considerable length (e.g., 4 m and more) and the illustrated embodiment is for purposes of discussion only. The HCE 20 includes a steel absorber tube 22 through which the heat transfer fluid flows. A common outside diameter for such an absorber tube is around 70 mm; however this is not a limitation. The outside surface of the absorber tube 22 typically includes a solar selective absorber surface. Further, the HCE 20 includes an annular glass envelope 24 concentrically disposed about the absorber tube 22. A common outside diameter for such a glass envelope is 115 mm; however this is not a limitation. The increased diameter of the glass envelope defines an annulus 26 between the inside surface of the glass envelope 24 and the outside surface of the absorber tube 22. The annulus 26 is evacuated to reduce heat losses at high operating temperatures and to protect the solar selective absorber surface from oxidation. As further shown, the ends of the illustrated HCE 20 include bellows 28, which accommodate thermal expansion differences between the steel absorber tube and the glass envelope.

FIG. 3A illustrates the concentration of sunlight energy/rays on the absorber tube 22 to heat the tube and the heat-transfer fluid therein. The parabolic reflector (not shown) reflects sunlight rays onto the HCE 20, which is disposed at the focal point of the reflector. As shown, a majority the reflected sunlight rays impinge on the absorber tube 22. However, due to mirror surface imperfections, sunlight tracking misalignments and/or other optical-mechanical phenomena, some of the reflected sunlight rays bypass the HCE without contacting the absorber tube 22. That is, some spillage occurs due to the imperfection of the focal point of the parabolic reflector. Such spillage reduces the overall efficiency of the HCE. Accordingly, the presented inventions are directed to an external or secondary reflector/concentrator that is attachable to the outside surface of an HCE (e.g., glass envelope 24), which captures reflected sunlight rays from the parabolic or primary reflector/concentrator that would normally be lost via spillage.

FIGS. 4A and 4B illustrate first and second perspective views of an external concentrator 40 as applied to an HCE 20. More specifically, these figures illustrate the external concentrator 40 as disposed about an outside surface of the glass envelope 24 of the HCE. FIGS. 5A and 5B illustrate a cross-sectional end view of the external concentrator 50 and the external concentrator as applied to the HCE 20, respectively. As shown, the external concentrator 40 includes a series of reflective ribs 50 that extend substantially parallel to the parabolic trough and its focal line once the external concentrator 40 is attached to the HCE 20. In the illustrated embodiment, the external concentrator 40 includes two sets of reflective ribs 50, which are separated by a reflective shield 60.

As shown, each of the ribs is an elongated element that is substantially rectangular in cross-section having two ends/edges and two opposing side surfaces. However, it will be appreciated that in further embodiments the ribs 50 may be shaped (e.g., curved, parabolic, cusp etc.). In any arrangement, the ribs will typically each have an end/edge surface that may be disposed along the length of the HCE 20. However, in various embodiments the ribs may be spaced above the surface of the HCE 20. The cross-sectional height of each rib, extending radially outward from the surface of the HCE 20, permit gathering of stray and misaligned reflected light rays while allowing properly directed light rays to pass into the HCE. In this regard, one or both side surfaces of each of the ribs 50 forms a reflector that allows for capturing stray and misaligned reflected light rays, which may then be re-directed onto the absorber tube 22 within the evacuated glass envelope 24. To redirect the stray reflected light rays, one or both side surfaces of the ribs is a partially reflective surface, which may be formed of, for example, reflective polished aluminum or specially coated reflective metal. Alternatively, a reflective film may be applied to the ribs 50.

The redirection of the stray and misaligned light rays by the ribs 50 is at least partially illustrated in FIGS. 3A-3D. As noted above, FIG. 3A illustrates the spillage of light rays that are reflected by the parabolic reflector but fail to contact the absorber tube 22. FIG. 3B illustrates an end-view of the external concentrator 40 and FIG. 3C illustrates the external concentrator 40 as applied to the HCE 20. FIG. 3D illustrates the reflected light rays of FIG. 3A as redirected upon the attachment of the external concentrator 40 the outside surface of the HCE 20. As shown, the ribs 50 which extend radially outward allow for capturing and redirecting a portion of stray light rays back onto the absorber tube 22. Further, due to the substantially radial alignment of the ribs 50, the ribs do not interfere with incoming light rays, which are properly focused on the absorber tube 20. That is, even if properly reflected incoming light rays contact the ribs, they are most commonly redirected to another point on the absorber tube 22.

While the ribs 50 provide the ability to capture some additional light rays which would otherwise spill past the HCE 20, it is been recognized that additional spilled light rays may be recaptured by the use of the external reflective shield 60. The illustrated embodiment of the reflective shield 60 is a corrugated element that is adapted for disposition on a radial outside portion of the HCE 20. More specifically, the reflective shield is disposed on the outside surface of the glass envelope 24 on the side of the glass envelope that is opposite of the vertex of the parabolic reflector. Referring again to FIG. 3D, it is shown that the inclusion of the reflective shield 60 significantly improves the redirection of spilt light rays back onto the absorber tube 22.

As noted above, the disclosed embodiment of the external concentrator 40 utilizes a pair of rib sets 50 that are separated by a reflective shield 60. The size and orientation of each of these elements may be varied. For instance, the number of the rib reflectors of each rib set may be varied based on physical parameters of the system with which they are used. Commonly, a height of the ribs in the radial direction will be between about ½ cm and about 3 cm. However, other sizes are possible and considered within the scope of the presented inventions. For instance, the height of the ribs will vary based on the diameter of the HCE. Along these lines, the height of the ribs may be between about 1% and 40% of the diameter of the HCE. Further, it will be appreciated that the axial length of the ribs may be varied based on, for example, the length of an HCE on which the ribs will be placed. Likewise, the number and placement of the radial ribs about the outside surface of the HCE 20 may likewise be varied. Currently, it is believed that the location of the reflectors should extend from approximately 30° (i.e., 0) on either side of a reference line between the vertex of the parabolic receiver and a central axis of the HCE 20 to about 90° (i.e., φ) on either side of the reference line. However, these angles may be increased plus or minus 30°. See FIG. 5B.

To correctly position the ribs and reflective shield, the present embodiment of the external concentrator 40 utilizes wire cables 62 that are spaced along the length of the concentrator 40. See FIGS. 4A and 4B. As shown, the wires 62 extend through apertures 54 in the base of each of the ribs. To provide appropriate spacing between each of the ribs, an annular spacer 56 may be disposed between each adjacent pair of ribs. In this regard, the wire passes through the annular spacer which maintain a desired spacing between the bases of adjacent ribs. Further will be appreciated that the ribs may be equally spaced or different sets of ribs may utilize different spacing. In the present embodiment, the wires 62 also extend around the outside surface of the shield 60 to maintain its position on the HCE. Various spacers may be incorporated between the shield and the ribs and or that the wire may extend through one or more apertures may be formed within the shield. In any arrangement, the wires allow for conveniently attaching and detaching the device with the outside surface of the HCE.

FIG. 6 illustrates a non-limiting embodiment of an external concentrator 40 as applied to an HCE 20. More specifically, this figure illustrates the external concentrator 40 with reflective ribs 50 disposed in non-radial orientations. That is, the ribs (i.e., in cross-section) need not extend outward from the central axis of the HCE. As noted above, each of the ribs is an elongated element that is substantially rectangular in cross-section having two ends/edges and two opposite side surfaces. However, it will also be appreciated that in further embodiments the ribs 50 may be shaped (e.g., curved, parabolic cusp, etc.). In any arrangement the ribs 50 will each have an end/edge surface that may be disposed generally along the length of the HCE 20 or spaced above the surface of the HCE 20. In the illustrated embodiment, the cross-sections of at least some of the ribs 50, extend outward from the surface of the HCE 20 in a direction that is non-radial with the centerline axis of the HCE 20. For example, the orientation of the ribs 50 may be described by a rib axis 32 extending from a cross-section of the rib 50 and intersecting a center line 30 extending through the central axis of a cross-section of the HCE 20 and the vertex of the collector (not shown). The ribs 50 may be oriented such that the rib axis 32 intersects the HCE 20 at a point on the center line 30 other than the central axis of the cross-section of the HCE 20.

Accordingly, each rib 50 may have a rib axis 32 that intersects the center line 30 of the HCE 20 at the same point, wherein this point is located at some distance from the central axis of the HCE 20. In other embodiments, individual ribs 50 within a rib set, located on one side of the collector may be oriented a different angles relative to each other. For example a first rib 50 a or subset of ribs located on one side of the HCE 20 may have a rib axis 32 a that intersects center line 30 at a first location 100 a, and a second rib 50 b or subset of ribs located on the same side of the HCE 20 may have a rib axis 32 b that intersects center line 30 at a second location 100 b. Further, ribs 50 located on opposite sides of the HCE 20 may be oriented independently of each other. In such an embodiment (not shown) a first rib 50 located on a first side of the HCE may have rib axis 32 that intersects a center line 30 at a first point, and a corresponding second rib 50 located on the opposite side of the HCE may have a rib axis 32 that intersects the center line 30 at a second point.

In addition to varying the orientation of the ribs 50 in relation to the HCE 20, FIG. 6 also illustrates that individual ribs 50 may have different lengths. In certain embodiments, the length of any rib 50 may be 1% to 40% of the HCE 20 diameter, and different ribs 50 may have different lengths within this range. For example a first rib 50 a may have a first length that is 20% of the HCE 20 diameter and a second rib 50 b may have a second length that is 15% of the HCE 20 diameter. In certain embodiment each rib 50 may be chosen to have a different length. It would be understood that one skilled in the art could vary the length and orientation of each rib 50 independently of the other ribs to increase radiation received by the HCE 20. Moreover, as described above, various ribs 50 may also take on different shapes (e.g., a first rib may be rectangular and another rib 50 may be curved). The shape may also be varied with the length and the orientation to increase the radiation received by the HCE 20.

FIG. 7 illustrates an embodiment of an external concentrator 40 with ribs or brims 150 located proximate to the end portions (e.g., lateral edges) of the reflective shield 60. In this embodiment, the brims 150 are attached to the reflective shield at an attachment point axis 34. In these arrangements, the brims 150 have an end/edge surface connected to reflective shield at attachment point axis 34 that may be disposed along the length of the HCE 20. The brims may have different orientations in relation to the surface of the HCE 20. The orientations of the brims 150 may vary in relation to a cross-sectional radial line 36 extending from the central axis of the HCE 20 through the attachment point 34. In the embodiment shown, the brims 150 are rotated about the attachment point 34 running along the length of the HCE 20 and oriented at angle β from the cross-sectional radial line 36. Currently, it is believed that the orientation of the brims 150 could range from approximately parallel to the cross-sectional radial line 36 to about 90° (i.e., β) on either side of the radial line 36.

To further increase the amount of radiation received by the HCE 20, the brims 150 illustrated in FIG. 7 may be shaped (e.g., curved, parabolic cusps, etc.). Moreover, as noted above in relation to embodiments describing the ribs 50 in a spaced relation from the reflective shield (see FIG. 6), the brims 150 connected to the reflective shield 60 may also vary in length. It is further understood, that each brim 150 shown in the embodiment of FIG. 7 may be varied independent of the other brims. For example, a first brim may be oriented at a first angle (e.g., 20°), have a first length (e.g., 30% of the diameter of the HCE 20) and have a rectangular cross-section, and a second brim may be oriented at a second angle (e.g., 30°), have a second length (e.g., 20% of the diameter of the HCE 20) and have a curved shape.

As shown, the reflective shield 60 in FIG. 7 comprises a continuous surface and is adapted for disposition on the outside portion of the HCE 20. In this illustrative embodiment, the reflective shield 60 has a corrugated surface consisting of a plurality of rectangular cross-sectional sections (e.g., flat cross-sections) disposed to form alternating ridges and grooves. However, in other embodiments the reflective shield 60 may have a corrugated surface consisting of a plurality of shaped (e.g., curved surfaces, parabolic cusps, etc.) cross-sectional sections disposed to form alternating ridges and grooves.

FIGS. 8A and 8B illustrate first and second embodiments of the external collector 40 located in a spaced relationship to the HCE 20. More specifically, the external collector 40 is oriented at a distance Δ from the surface of the HCE 20, such that the surface portions of the reflective shield 60 are not in direct contact with the HCE 20. Currently it is believed that the external collector 40 should be located at approximately a distance of 0-40% of the HCE 20 diameter away from the surface of the HCE 20 (i.e., Δ=0-40% the diameter of the HCE 20). Further as noted above and shown in FIGS. 8A and 8B, brims 150 may be located on the ends of the spaced reflective shield 60. These brims 150 may vary in shape, length and orientation as described in relation to the previous figures. In addition, when utilizing a spaced reflective shield, it will be appreciated that ribs or rib sets may be applied to the surface of the HCE and/or spaced above the surface of the HCE.

The foregoing description has been presented for purposes of illustration and description. Furthermore, the description is not intended to limit the inventions and/or aspects of the inventions to the forms disclosed herein. Consequently, variations and modifications commensurate with the above teachings, and skill and knowledge of the relevant art, are within the scope of the presented inventions. Along these lines, different aspects of the inventions shown in different figures may be utilized in various combinations including combinations not explicitly shown. The embodiments described hereinabove are further intended to explain best modes known of practicing the inventions and to enable others skilled in the art to utilize the inventions in such, or other embodiments and with various modifications required by the particular application(s) or use(s) of the presented inventions. It is intended that the appended claims be construed to include alternative embodiments to the extent permitted by the prior art. 

What is claimed is:
 1. A solar concentrator configured for attachment to a generally cylindrical outside surface of a heat collection element (HCE) positioned along a focal line of a parabolic reflector, comprising: at least first and second elongated ribs each having, in cross-section, an edge surface adapted for disposition along a length of the HCE and at least one reflective side surface extending outward from the outside surface of the HCE, wherein said elongated ribs are radially spaced about the outside surface of the HCE.
 2. The solar concentrator of claim 1, wherein said first and second elongated ribs comprise a first set of elongated ribs and a second set of elongated ribs, respectively.
 3. The solar concentrator of claim 1, wherein each rib of each set of ribs is spaced from at least one adjacent rib when attached to the outside surface of the HCE.
 4. The solar concentrator of claim 3, further comprising: a reflective shield having a reflective surface disposed toward the outside surface of the HCE.
 5. The solar concentrator of claim 4, wherein said reflective shield covers between about 15° (i.e., ˜0.25 rad) and about 120° (i.e., ˜2.1 rad) of the outside surface of the HCE.
 6. The solar concentrator of claim 5, wherein said reflective shield is disposed on the outside surface of the HCE between said first and second elongated ribs.
 7. The solar concentrator of claim 4, wherein said reflective shield is corrugated.
 8. The solar concentrator of claim 4, further comprising: first and second brims attached to lateral edges of said reflective shield at first and second attachment points.
 9. The solar concentrator of claim 8, wherein said brims are, in cross-section, disposes at an angle +/−90° (β) on either side of a radial line extending through a centerline axis of the HCE.
 10. The solar concentrator of claim 9, wherein said brims, in cross-section, are: rectangular, parabolic; or curved.
 11. The solar concentrator of claim 2, wherein said first and second sets of elongated ribs are disposed on opposite sides of a reference line extending between a vertex of the parabolic reflector and a centerline of the HCE.
 12. The solar concentrator of claim 11, wherein each set of elongated ribs are radially spaced over about 15° (i.e., ˜0.25 rad) and about 120° (i.e., ˜2.1 rad) of the outside surface of the HCE.
 13. The solar concentrator of claim 1, wherein a height of said ribs, in cross section, is between about 1% and about 40% of a diameter of said generally cylindrical heat collection element (HCE).
 14. A solar concentrator configured for attachment to a generally cylindrical outside surface of a heat collection element (HCE) positioned along a focal line of a parabolic reflector, comprising: a reflective shield extending over an outside surface of the HCE, having a reflective surface disposed toward the outside surface of the HCE; at least first and second elongated brims extending from lateral edges of the reflective shield each having at least one reflective side surface extending outward from the outside surface of the HCE, wherein the elongated brims are connected to the reflective shield at first and second attachment points.
 15. The solar concentrator of claim 14, wherein the first and second elongated brims, in cross-section, extend from the HCE a length between 1-40% of the HCE diameter.
 16. The solar concentrator of claim of claim 15, wherein the brims, in cross-section, are: rectangular; parabolic; or curved.
 17. The solar concentrator of claim 16, wherein the first and second elongated brims are oriented at an angle relative to first and second cross-sectional radial lines, respectively, wherein the first and second cross-sectional radial lines extend from a central axis of the HCE through the first and second attachment points, respectively.
 18. The solar concentrator of claim 17, wherein the first and second elongated brims are oriented at an angle between 0-90° in either direction of the first and second cross-sectional radial lines, respectively.
 19. The solar concentrator of claim 14, wherein the reflective shield forms a corrugated surface comprising a plurality of alternating ridges and grooves.
 20. The solar concentrator of claim 19, wherein the corrugated surface, in cross-section, is formed from one of alternating: rectangular sections; parabolic sections; or curved sections. 